There is no center to the universe. Every point moves away from every other point, and the further you look, the faster things are moving away from you. It's like a piece of rubber being stretched in all directions equally. We don't know if the universe is finite or infinite, we just know that it looks like it's roughly the same everywhere, and every point will look like the center because everything else is receding.

On the dark matter/dark energy that @Son_Of_Hate dismissed out of hand: This is a fundamental misunderstanding of how these concepts came to be.

Up until fairly recently, astronomy and cosmology operated under a simple assumption: all the stuff that we (can) see is all the stuff that exists. Which was partly a assumption out of necessity, since all information we get from astronomy is in the form of light, and partly a well justified assumption since all matter that we know of interacts with light. (I'm glossing over neutrinos now because they're irrelevant to the story.)

After taking more and more measurements, it appeared that our observations we did using light seemed to be missing something. Things weren't moving at the right speeds, or were appearing heavier than they should be. So this can mean one of two things:
1. There is matter there that we can't see (the dark matter hypothesis)
2. Our current understanding of gravity is incomplete (Modified Newtonian Dynamics - MoND)

Both the dark matter hypothesis and MoND have been studied extensively, and it turns out that only the dark matter hypothesis properly corresponds to observations without the necessity of tweaking parameters for every single case and every single scale. It works for galaxies, for clusters, for superclusters, for cosmological models. MoND needs to be reparametrized for every scale and usually doesn't transfer well from case to case. So currently, the dark matter hypothesis is preferred over MoND. People are still trying different variations of modified gravity, including modified versions of general relativity, but so far nothing has come up that actually works, and most modifications tend to break general relativity in very fundamental ways. It's not discarded entirely, but the evidence in favor of dark matter gets stronger every year.

In case of dark energy, this is an idea that dates back to Einstein. If the universe itself has a certain baseline energy density (the cosmological constant), then this would work as a sort of expansive force. From observations it appears that the unvierse is not only expanding, it is doing so at an accelerating pace. This corresponds very well to the idea of a cosmological constant which is driving accelerated universal expansion. However, it's extremely difficult to get reliable and precise measurements of this, and our theoretical models which should predict a sensible value for the cosmological constant seem to be off by a lot. So there are a lot of unknowns surrounding dark energy. So far, it seems like dark energy is real and we have a fairly good estimate of its strength. But better observations are necessary, and it would probably help a lot if we could develop a working theory of quantum gravity. Only time will tell.

One final note: don't believe amateurs like @Son_Of_Hate who think they're smarter than experts for dismissing things they do not understand. Don't be like that. It's annoying and all you're doing is embarrassing yourself. If you don't understand something, ask and you might learn. Dismiss and you will remain ignorant.